This invention relates to a structural material suited to be used in racket frame for tennis, squash, badminton or the like, pole for tent frame, pipe for structural material, buried earth pipe, block, fishing rod or the like, and a method of fabricating the same.
Fiber reinforced composite materials conventionally used as structural materials for building materials or sports articles include those prepared by adding short fiber reinforced materials to a thermoplastic resin used as the matrix, or other prepared by adding long filament reinforced materials to a thermoset resin used as the matrix.
These materials, however, involve certain problems in the forming property, strength, heat resistance and fatigue characteristic, and superior fiber reinforced composite materials have been demanded.
For example, reinforced plastic poles are recently sold on market as the pole for mountaineering tent, but they have their problems as stated below, and they are presently used partly only in a field of less rugged conditions such as picnic tent.
That is, in the composition of these reinforced plastic poles, the reinforced fibers are composed of glass, carbon, or aromatic polyamide, and the matrix is made of epoxy, polyester or other thermoset resin. The most widely employed method of fabricating reinforced plastic poles is the so-called xe2x80x9cdrawing pipexe2x80x9d method which is to impregnate glass fibers with polyester resin, and continuously lead into the hardening bath to be hardened. In other versions, fibers are processed by filament winding (F/W) process, not limited in the axial direction, or ABS resin or other thermoplastic resins are used as the padding or covering material, but in any event the matrix of reinforced fibers is always a thermoset resin such as polyester and epoxy.
These thermoset resins are, by nature, brittle, and are easily broken when bent sharply like a tent pole. Or if the rigidity is lowered to allow a large deflection in practical usage, problems occur in the aspects of habitability or resistance to wind pressure. In particular, when several poles are joined together, stress is concentrated at joints, and most breakdowns originate in the joint. However, in the continuous drawing method, since its section is uniform, it is difficult to reinforce only the joint part integrally. Or in the case of F/W process, it is difficult to vary the complicated joint shape or outside diameter, and there are serious problems such as fluctuations of dimensional precision and performance derived from the nonuniformity of tape wrapping.
On the other hand, in the composition of fiber reinforced plastic used as the material of golf club shaft, the reinforced fibers are composed of carbon, glass, aromatic polyamide, boron or the like, and the resin matrix is made of epoxy, polyester or other thermoset resin. All of them share a main feature of lightness of weight as compared with metallic shaft, and also provide the following merits.
1) The head speed when swinging increases.
2) Even a less powerful golfer can swing easily.
Fabrication methods of this kind of shaft are roughly divided into two types as follows.
(1) Filament winding (F/W) method
A continuous filament is impregnated with resin, and is wound on a mandrel at a specified angle in the axial direction.
(2) Sheet winding method
A cloth impregnated with resin is wound on the mandrel.
In both (1) and (2), after forming on the mandrel, the material is wrapped by heat-shrink tape, and is heated in a hardening furnace.
In these methods, however, the dimensional precision when forming the material is not sufficient, and the pressure when hardening depends on the tightening force of the wrapped tape, so that the dimensional precision of product is limited. Besides, since marks of wrapping tape are left over on the product surface, it is necessary to finish the surface by buffing with centerless grinder or the like, so that part of surface fibers is shaved off. And it is also difficult to vary the complicated shape or outside diameter noncontinuously, and the degree of freedom of design is limited. Furthermore, thermoset resins of epoxy and polyester are more brittle, and may be broken when an impact is applied.
In addition, the following two types are known as the composition of fiber reinforced plastics for racket frame for ball games.
(1) Continuous fiber/resin matrix type
(2) Short fiber of chopped fiber/resin matrix type
In type (1), epoxy, polyester or phenolic thermoset resin is used as the resin matrix, and it is impregnated in continuous filament, and heated and pressurized, so that the resin is hardened and molded into a desired shape.
In type (2), the reinforced members are composed of fiber reinforced members of short discontinuous length randomly dispersed in the resin matrix, and this resin may be either thermoplastic or thermosetting. As the thermoset material, the one shown in (1) is used, and as the thermoplastic material, for example, nylon, polycarbonate, polyphenylene oxide, acetal and other so-called industrial thermoplastics are used. As the molding method, mainly injection molding is employed.
On the other hand, as the characteristics required in rackets, usually, toughness, rigidity and resilience are known. As for toughness, since the toughness of matrix resin of type (1) is inferior, expensive carbon fibers or other reinforced fibers are used usually by 60 to 70 wt. % in order to obtain a required toughness. Since this is an easy method of obtaining a required strength and desired shape, this method is employed in most existing tennis racket frames using reinforced plastics.
In the case of (2), usually, considering the moldability, in particular, fluidity at the time of injection, the molecular weight of matrix resin is kept low. The fiber content is about 30 wt. %, and the fiber length is mostly less than 1 mm (0.2 to 0.3 mm) after pelletizing and injection molding. Since the matrix resin is not high in molecular weight and the length of reinforced fibers is extremely short, improvement of mechanical strength in this composition is not expected. Therefore, if such racket strung with guts is kept in an automobile trunk, for example, and its internal temperature exceeds 80xc2x0 C., it is highly possible that the frame may be deformed or broken during use.
To compensate for this defect, it is consequently necessary to increase the wall thickness of racket frame, but since the total weight increases, it is not so practical.
Recently, meanwhile, as sports are becoming popular as a pastime, consideration to sports injuries is required. For example, according to a certain polling, about one third of tennis players claimed to have xe2x80x9cexperienced pain in the elbow.xe2x80x9d This is known as tennis elbow, and the player feels pain suddenly in the elbow of the racket holding side without any specific cause. In a racket inferior in vibration absorption property, it is said that the vibration of hitting a ball is transmitted to the elbow to damage the humerus epicondylus. In the continuous filament/resin matrix type (1) which is in the mainstream of the present racket frame materials, since the commonly used epoxy resin and polyester resin are inferior in impact absorption, it is considered that the vibration characteristic be interior.
Incidentally, as technical reports about industrial materials using nylon resins (which is similar to that used in this invention), for example, xe2x80x9cNylon RIM Development for Automotive Body Panelsxe2x80x9d (SAE Technical Paper Series 850157, 1985), and xe2x80x9cNylon 6 RIMxe2x80x9d (American Chemical Society, 1985) are known, and also an article relating to terminal amine polyether RIM (SAE Technical Paper Series 850155, 1985), an article relating to the future of RIM in America (American Chemical Society, 1985), and an article relating to RIM monomer casting (xe2x80x9cPlastics Technology,xe2x80x9d May 1965 issue) are available, but nothing is mentioned about long filament reinforced products in these papers.
The present invention is devised in the light of the above background, and it is hence a primary object of this invention to present a structural material which is lightweight, excels in strength and flexural modulus, and is large in the degree of freedom of designing of shape and material as compared with that of conventional materials.
The structural material of this invention (a first aspect of the invention) is characterized by a polyamide resin reinforced by continuous fiber and/or long filament reinforcing material, used as the base material to form the structure. The method of fabricating the structural material of this invention (a second aspect of the invention) is a method of fabricating a structural material composed of polyamide resin reinforced by continuous fiber and/or long filament reinforcing material, in which the long fiber and/or long filament reinforcing material is arranged preliminarily in a desired shape and put in a mold, and a molten w-lactams containing polymerization catalyst and initiator is poured into the mold, and it is heated to obtain polyamide resin by monomer casting method, thereby forming a structural material.
The monomer used in this invention, w-lactams, may include the following examples: xcex1-pyrrolidone, xcex1-piperidone, e-caprolactam, w-enantolactam, w-caprilolactam, w-peralgonolactam, w-decanolactam, w-undecanolactam, w-laurolactam, their c-alkyl substitute-w-lactam, and mixture of two or more kinds of w-lactams. However, what is advantageous industrially is xcex5-caprolactam or w-laurolactam. And w-lactams may contain, if necessary, modifying components (soft components).
The molecules of a soft component possesses in molecule a functional group reacting with an initiator used, and it is a component of low Tg, and usually polyether or liquid polybutadiene possessing functional group is used.
A commercial material used in this invention may be, for example, UX-21 which is a nylon RIM material manufactured by Ube Industries, Ltd. It is composed of component A made of alkali catalyst and caprolactam, and component B made of prepolymer containing soft component and caprolactam.
As the anionic polymerization catalyst used in this invention, sodium hydride (NaH) is preferable, but also other sodium, potassium, lithium hydride and known w-lactam polymerization catalysts may be used. The content is preferably in a range of 0.1 to 5.0 mol % of w-lactam.
As the polymerization initiator, N-acetyl-xcex5-caprolactam is used, but other applicable examples are triallylisocyanurate, N-substitute ethylene imine derivative, 1,1xe2x80x2--carbonyl visazilidine, oxazoline derivative, 2-(N-phenylbenzimidoyl) acetoanilide, 2-N-morpholino-cyclohexene-1.3-dicarboxysanilide, known isocyanate, carbodimide and similar compounds. The content of the initiator is preferably in a range of 0.05 to 1.0 mol % of w-lactam. The methods of its addition include:
(A) A method of directly adding and mixing to w-lactam solution containing anionic polymerization catalyst;
(B) A method of mixing w-lactam solution containing anionic polymerization catalyst and another w-lactam solution containing polymerization initiator; and
(C) A method of adding together with anionic polymerization catalyst preliminarily to the solid or liquid w-lactam. Any method may be employed.
The polymerization temperature is generally preferable in a range of 120 to 200xc2x0 C., but it is also possible, for special purposes, to polymerize under 120xc2x0 C. or over 200xc2x0 C.
As the continuous fiber which is a reinforcing material depending on the applications, carbon fiber, aramide fiber, glass fiber, alumina fiber, silicon carbide fiber, steel wire, amorphous metal fiber and/or their hybrid may be used in a state of cloth, sleeve or roving. Continuous fiber and/or long filament of fully aromatic aramides with a modulus of elasticity in tension of 3,000 to 30,000 kg/mm2, or more preferably, 5,000 to 15,000 kg/mm2, are suitable.
The continuous fibers and/or long filaments are placed in the mold, for example when fabricating a pipe-shaped structural material, by winding a necessary amount around the core, or covering the core as sleeve. To obtain a block-shaped product, it may be preliminarily set in the mold.
In this invention, since the monomer casting method is employed, there is no limitation to the molecular weight in consideration of the molding processability, and a polyamide resin of high molecular weight is obtained, so that the strength, elasticity and thermal distortion temperature are high. So the thickness of the structural material may be reduced, and the weight may be light.
As the material of the thermoplastic resin reinforced by long filaments, a so-called stampable sheet is known, and as its nylon version, a sheet composed of nylon resin and continuous glass fiber mat may be considered as an example, but when obtaining a molded product of desired shape by using such stampable sheet, the following problems exist. That is, when a stampable sheet is used as molding material, it is necessary to handle the heated and melted material sheet outside the mold, and at this time the temperature is as high as 200 to 350xc2x0 C., and the molten material sheet is very soft and extremely hard to handle. Besides, a heating equipment using far infrared rays is required, and the facility cost is high. Furthermore, the press pressure at the time of molding is very high, about 100 to 300 kg/cm2, and the mold and other facilities are expensive. Still more, the reinforcing fibers form relief patterns on the molded product surface, or air bubbles mixed in the heating process of material cannot be forced out completely when molding and are left over on the surface, and the surface finished state is not favorable. Or it is difficult to form a thin or a complicated shape.
Therefore, the method of this invention by monomer casting process seems far more excellent.
In this invention, moreover, since a tough polyamide resin is used instead of brittle thermoset resin, the content of reinforcing fiber may be small, and in particular by using continuous fiber and/or long filament it is possible to reinforce further and decrease the content of reinforcing fibers, so that economy and light weight may be achieved together. In addition, the excellent vibration attenuating characteristic of the polyamide resin used in the matrix resin becomes more notable because the content of reinforcing fibers is small, so that the racket frame, shaft for ball games, and other structural materials light in weight and excellent in durability and appearance may be obtained.
The structural material by this invention is excellent in lightness of weight, strength, flexural modulus, vibration attenuation characteristic and other properties, because tough polyamide resin, instead of brittle thermoset resin, is used as the matrix resin, and it is further reinforced by continuous fiber and/or long filament reinforcing material.
This structural material is easily formed in a desired shape by the fabricating method according to the second invention, in which the fiber reinforcing material is preliminarily placed in a desired shape, and the matrix resin is added to it by monomer casting.
The conventional fiber reinforced composite materials were fabricated by a premix method in which the reinforcing material was premixed with the resin and then formed, and it was difficult to form in a complicated shape generally, while, in this invention, it is easy to form and the degree of freedom of design is great.